Studies on the Helminth Fauna of Alaska. XXI. Taxonomy, Morphological Variation, and Ecology of Diphyllobothrium ursi n. sp. Provis. on Kodiak Island

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Abstract
Cestodes of the genus Diphyllobothrium Cobbold, 1858 are commonly harbored by bears on Kodiak Island, in the Gulf of Alaska. During months of summer and early fall these mammals feed largely on salmon which spawn in great numbers in island streams. Circumstances suggest that the bears ingest plero-cercoid larvae in salmon. Adult cestodes were secured at the autopsy of bears, Ursus arctos middendorffi Merriam on Kodiak Island. Plerocercoid larvae of Diphyllobothrium sp. were collected from spawning or spawned-out red salmon, Oncorhynchus nerka (Walbaum) and fed to six young black bears, Ursus americanus Pallas. Cestodes morphologically identical with those taken from bears on Kodiak Island developed in the experimentally infected black bears. A comparative study of adult cestodes disclosed considerable morphological variation in members of the genus Diphyllobothrium. An evaluation of the characters regularly considered in differentiating species of Diphyllobothrium was made. Shape of the scolex depended upon the state of expansion or contraction at the time of fixation while size of the scolex was relatively uniform irrespective of strobilar size. It was considered that average dimensions might, in some cases, have value in differentiating species of Diphyllobothrium. The author found that the presence of a neck, defined as the unsegmented portion of the strobila following the scolex, depended upon the age of the strobila in some spp. of Diphyllobothrium. It was concluded that the presence or absence of a neck has no diagnostic significance in the cestodes from Kodiak Island bears. Information on growth rate, obtained in this study through feeding experiments, indicated that differences in host spp. may influence considerably the strobilar development of a given sp. of Diphyllobothrium. It was concluded that size of the strobila per ae has little diagnostic value. Distribution of the testes were fairly uniform for a given species; however, number of testes was subject to a wide range of variation. Much variation was seen in the form and extent of posterior and anterior forms of the ovarian lobes; no specific shape of the ovary was defined as being characteristic for this cestode. Number of uterine coils was not considered diagnostic in the writer''s material. Distribution of vitelline glands had taxonomic value but enough material must be available to determine first the range in variation. Size of Mehlis'' gland was of little diagnostic value. Differences of reasonable degree in size and proportions of eggs were of value in distinguishing spp. of Diphyllobothrium but this can be evaluated only after confirmation of specific status of numerous spp. now regarded as valid. Size of ootype, the receptaculum seminis, and of the genital apertures, had little systematic value in differentiating the adult cestodes. Differentiation of the plerocercoid larvae of certain spp. of Diphyllobothrium was discussed. Four points were considered in attempting to characterize these larvae: (1) localization in body of host; (2) sp. of host; (3) external morphology; (4) microscopic anatomy. It was concluded that the Kodiak Island cestode perhaps can be characterized according to structural and ecological peculiarities of both the adult and the plerocercoid stages. Since, according to present knowledge, it was not possible to confirm that this form is specifically identical with any previously-described sp., the name D. ursi n. sp. provis, was proposed. Adult and plerocercoid larva are described. A discussion of ecology of D. ursi is included.